1. Field of the Invention
The present invention relates to a unidirectional surface acoustic wave transducer, for use in a surface acoustic wave filter or the like, and more specifically, the present invention relates to a surface acoustic wave transducer in which modification of the directivity of reflectors achieves greatly improved transmission characteristics. The present invention also relates to a transversal-type surface acoustic wave filter having such a unidirectional surface acoustic wave transducer.
2. Description of the Related Art
Recently, with advanced digitalization in the telecommunication area, there has been a demand for a surface acoustic wave filter having reduced insertion loss and good phase linearity. Conventional types of surface acoustic wave filters include a transversal-type surface acoustic wave filter and a resonator-type surface acoustic wave filter.
The conventional transversal-type surface acoustic wave filter has a problem in that insertion loss is large, although phase linearity is satisfactory.
On the other hand, the resonator-type surface acoustic wave filter has a problem in that phase linearity is unsatisfactory, although insertion loss is small.
Accordingly, as a surface acoustic wave filter having both reduced insertion loss and satisfactory phase linearity, there has been proposed a surface acoustic wave filter, which includes a single-phase unidirectional surface acoustic wave transducer (hereinafter referred to as a SPUDT). Various structures of a conventional type of SPUDT are presented, such as an EWC type, a reflection-bank type, a FEUDT. See for example, C. S. Hartmann and B. P. Abbott: Proc. IEEE Ultrasonics Symposium (1989) pp. 79-89; M. F. Lewis: Proc. IEEE Ultrasonics Symposium (1983) pp. 104-108; K. Yamanouchi and H. Furuyashiki: Proc. IEEE Ultrasonics Symposium (1984) pp. 68-71, Japanese Examined Patent Publication No. 2-45364, etc.
In such a type of SPUDT, directivity is provided by shifting the reflection center of the reflector only by .lambda./8 (.lambda. represents wavelength of a surface acoustic wave in a center frequency) from the excitation center of an interdigital electrode transducer (hereinafter referred to as an IDT) in the direction of propagation of the surface acoustic wave, so that bi-directional loss, which is typical of the transversal-type surface acoustic wave filter, is reduced.
Currently, however, an IF filter used in telecommunication devices, particularly, in CDMA (Code Division Multiple Access) devices, is required to have higher selectivity, and a rapid change in attenuation in a region between a pass band and a block band. To this end, it is necessary only that the number of pairs of electrode fingers in an IDT be increased. Meanwhile, as electronic devices are miniaturized, surface acoustic wave filters that are to be mounted therein must be made smaller. As a result, it is difficult to increase the number of pairs of electrode fingers in the IDT.
Since the number of pairs of electrode fingers in the IDT cannot be increased by much, the conventional type of surface acoustic wave filter makes only a slight change in attenuation between the pass band and the block band, failing to satisfy the requirements for the filter characteristics. In addition, the conventional filters present problems in that manufacturing tolerance is undesirably restricted thereby reducing manufacturing yield.